Updates are not incremental. You get the whole new thing, even if the diff would have been 1 character. Combine that with the fact that AIUI, DMGs and XIPs are stuck with deflate/gzip or bzip2, all of which are slow by modern standards, and you're on for some pain. Also, macOS may not have enough entries in its inode cache for the number of files there are in Xcode (although I haven't looked how many there are, maybe it's fine)
I actually had a poke around the XIP format after I was particlarly annoyed after the XCode update.
For XCode: The XIP contained some kind of DEFLATE'd index I didn't bother with. Most of the meat was an uncompressed "PBXZ" stream, which contained various LZMA compressed CPIO chunks.
